The melting or refining of iron or steel is an energy intensive process in that very high temperatures are required to attain and maintain a molten metal bath. Accordingly, there has occurred in the steelmaking industry a continuing effort to use the available energy more efficiently. One such practice which is widely used in steelmaking is post combustion. In a steelmaking molten metal bath, oxygen reacts with carbon to form carbon monoxide which bubbles up and out of the bath. Post combustion is the injection of secondary oxygen into the steelmaking vessel to react with the generated carbon monoxide to form carbon dioxide. In addition, hydrogen and/or hydrocarbons may also be present or formed, for example, by the breakdown of contaminants associated with steel scrap or the partial oxidation of methane gas or coal, and such hydrogen and/or hydrocarbons may react with secondary oxygen in a post combustion reaction to form water and carbon dioxide. These reactions are exothermic and much of the resulting released energy is gainfully employed as heat in the steelmaking vessel.
Significant work has been done in recent years to improve post combustion practice in steelmaking. In U.S. Pat. No. 4,599,107--Masterson, there is disclosed a method for carrying out post combustion in subsurface pneumatic steel refining wherein secondary oxygen is injected through a lance into the headspace above the bath surface. The lance height above the bath and/or the velocity of the injected secondary oxygen are adjusted to satisfy a defined relationship to enable the efficient practice of the post combustion.
U.S. Pat. No. 5,065,985--Takahashi et al. teaches an improved method for smelting reduction of iron ore wherein post combustion oxygen is injected into the slag layer, which is also strongly stirred, so that the post combustion occurs mainly within the slag layer. The paper by Ibaraki et al. entitled Development of Smelting Reduction Of Iron Ore--An Approach To Commercial Ironmaking, I&SM, 12, 1990, pp. 30-37, teaches a similar post combustion practice wherein the oxygen molecules remain in the jet until they collide with the foamy slag and are transferred into it. The oxygen burns the carbon monoxide and hydrogen in the foam. Different effects are attained by the degree to which the secondary oxygen jet penetrates or agitates the slag.
An electric arc furnace is employed to melt metal such as steel for subsequent refinement. An electric arc furnace generally comprises a relatively short, wide, cylindrical enclosure having a relatively wide and shallow hearth within which metal is melted. It has one or more electrodes, generally three, which pass through the furnace roof and are centrally arranged within the furnace to provide electrical energy to heat and melt the metal. Post combustion has also been used in electric arc furnace practice such as disclosed in European Patent No. 257,450--Brotzmann et al. wherein the top injected oxygen is injected into the upper region of an electric arc furnace through a plurality of top blowing devices oriented so as to provide coverage over the entire furnace for the uniform combustion of carbon monoxide. U.S. Pat. No. 4,986,847, Knapp et al. teaches the provision of oxygen in a region adjacent the molten metal within the slag layer.
The use of electric arc furnaces is increasing as more scrap metal is being recycled. Thus any improvement in the operation of electric arc furnaces would be useful and desirable.
Accordingly, it is an object of this invention to provide a method for employing post combustion in electric arc furnace practice which will improve the operation of electric arc furnaces.